The present invention is for a system utilizing a microprocessor-controlled current sensor, which functions in the capacity of a fuse or circuit-breaker for an electric motor.
There are essentially two advantages in which the inventive system is an improvement over similar "state of the art" motor protection systems: (a) in the ability of the invention to "learn" a start-up load-current profile for the particular motor being monitored, therefore not having to manually search for an appropriate tripping point, and (b) in the ability of the invention to monitor a motor load-current "non-invasively." In essence, the present invention customizes itself or adapts to each motor being monitored.
The invention monitors the current drawn by the electric motor by placing two sensors about the line or lines through which current is provided to the electric motor. Although this type of non-invasive mechanism is not new per se for detecting current, it is believed, however, that it has never been used in this type of peak current protecting application, or for the purpose of protecting electric motors, either AC or DC.
The method of non-invasive testing of motor currents in accordance with the invention, as aforementioned involves the placement of two magnetic-field sensitive current sensing elements adjacent or about the lead-wires of the motor. The sensing elements are positioned about the lead wires, one for each direction of current. As the current flowing through each line increases in one direction, the inductance of the current sensing elements for that direction decreases. A circuit connected to the current sensing elements measures and transforms the inductance into a square wave, whose frequency is inversely proportional to the magnitude of the inductance. The number of pulses in a given time period is counted, yielding a number proportional to frequency. The circuit of the system contains a microprocessor controller, which calculates the amount of current in the motor lines by applying a formula stored in memory to the measured frequency.